This invention relates to reflector antennas for spacecraft use, and more particularly to shaped reflectors for providing substantially uniform Earth illumination from a geosynchronous spacecraft.
Spacecraft transponders are widely used for communications and television broadcasting. Some communications systems use a constellation of low-Earth-orbit (LEO) spacecraft travelling in elliptical orbits, such that some of the spacecraft are always available to serve various portions of the Earth""s surface. These type of orbits require that a communication system provide for handoff or switching among the spacecraft as they succeed each other in providing service to a particular location. Other communications spacecraft and television broadcast spacecraft occupy geosynchronous orbits, in which the spacecraft appears to be at a constant location as seen from the Earth""s surface. Such geosynchronous orbits eliminate the need for switching or handoff, but require a theoretical orbital altitude of about 35,786 kilometers or 22,237 miles, which is substantially greater than the altitude of those LEO spacecraft which provide service to a region. Consequently the payload of a geosynchronous spacecraft must provide a greater effective isotropic radiated power (EIRP) than a LEO spacecraft in order to provide the same signal strength at the Earth""s surface.
Among the types of antennas used by geosynchronous spacecraft are direct-radiation horns. Ordinary horns tend to have H-plane radiation patterns which are more broad or wider than the E-plane radiation patterns when operated in the dominant mode, because of the characteristics of the H-plane aperture distribution. Since the Earth as seen from space is a circular disk, the differing E- and H-plane beamwidths of a conventional horn for direct radiation result in some energy being xe2x80x9clostxe2x80x9d into space past the horizon in the H-plane if the E-plane pattern is proper, or result in insufficient coverage near the horizon in the E-plane if the H-plane coverage is adequate. Specialized horns can be used which are optimized for equal E- and H-plane coverage.
From a geosynchronous spacecraft orbiting at 35,786 km from the surface of the Earth, the Earth""s radius of about 6,378 km results in an effective subtended angle of about 9xc2x0, corresponding to a subtended diameter of about 18xc2x0. Also, the horizon as seen from the geosynchronous spacecraft is more distant than the nadir by about 5893 km. Consequently, the path length between the spacecraft and the nadir is about 35786 km, while the path length between the spacecraft and the horizon is about 41,677 km. This difference in distance corresponds to a signal level difference at the horizon which is about three or four dB less than that at the nadir, assuming isotropic radiation from the spacecraft. Antenna Handbook by Lo and Lee, published 1988 by Van Nostrand Reinhold Company describes a direct-radiating circular horn array including a large multimode central horn surrounded by eight smaller horns, all fed with a beamformer, for generating an Earth-disk-coverage beam having a beam-center gain of about 15 dB, and a xe2x80x9cbeam-edgexe2x80x9d gain of about 18 dB, which provides substantially constant power or energy distribution over a hemisphere of the Earth. Thus, the power flow per unit area tends to be about the same at the nadir and the horizon, and at locations inbetween. Horn radiator arrays tend to be bulky and heavy, and the beamformers required for such an array make the horn array undesirable for spacecraft applications. Lo et al. also mention corrugated horns for Earth-disk coverage, but indicate that they tend to be high in cost, difficult to fabricate, and relatively heavy.
Improved antennas are desired.
According to an aspect of the invention, a reflector antenna suited for providing Earth coverage from a geosynchronous spacecraft comprises a feed radiator for producing a directive feed beam, and a reflector intercepting the feed beam. The reflector is shaped for generating a generally axially symmetric reflector antenna beam about a beam axis, which symmetric reflector antenna beam has a directivity or theoretical gain in the range of about fourteen or fifteen db at the beam center, with the directivity increasing to about eighteen dB at angles of about 9xc2x0 from the axis.
In a particular embodiment of this aspect of the invention, the reflector has a surface contour in at least a first plane which includes a convex center portion which is convex in a center region of the reflector as seen from the feed. The reflector also has a convex edge portion adjacent the edge of the reflector, which convex edge portion is convex as seen from the feed. The reflector also has a further concave portion, as seen from the feed, in a region lying between, and connecting, the convex center and edge portions. In one version of this embodiment, the feed is offset from an axis of the reflector and lies in a second plane orthogonal to the first plane. In this version, the reflector has a surface contour in the second plane in which the convex center portion has a maximum projection which is offset in the same direction as the feed is offset from the axis of the reflector. In another avatar, the shaped reflector is based on an underlying parabolic reflector, and the center portion of the shaped reflector is depressed below the corresponding contour of the underlying parabola, while the outer edge of the reflector is raised above the corresponding contour of the underlying parabola. In this avatar, the progression from negative distance or depression to positive distance or projection above the underlying parabolic reflector is monotonic except possibly at the very edge of the reflector.
In another manifestation of the invention, the reflector subtends only that portion of the directive feed beam which exhibits power levels greater than about 13 dB below the peak power level exhibited by the feed antenna beam, for providing a particular level of sidelobes of said reflector antenna beam. In another version of this manifestation which provides lower sidelobes than the 12-dB intercept, the reflector subtends only that portion of the directive feed beam which exhibits power levels greater than about 19 dB below the peak power level exhibited by the feed antenna beam.